(1) Field of the Invention
This invention relates to a multiple electrode submerged arc welding method and more particularly to a submerged arc welding method capable of drastically increasing the welding efficiency.
(2) Description of the Prior Art
The submerged arc welding method is a method in which the welding is carried out while the atmosphere of the arc is seated by a flux and hence, can use a greater welding current in comparison with a MIG welding method. Its current density can also be enhanced by several times higher than that in manual welding. In addition, since the fusing speed of the wire and the depth of penetration can also be increased markedly, the method has the advantage that welding can be accomplished for a material having a considerably large thickness. On the contrary, however, the submerged arc welding method is not free from the disadvantage that since the value of current to be used is high, heat input applied to a material to be weld is large and the range of embrittlement tends to increase along with expansion of the heat affected zone. When the submerged arc welding method is employed for a structure for which low temperature toughness or the like is a requisite, therefore, it has been a customary practice in the art to employ a multi-layer build-up welding method in order to disperse heat input and to narrow the heat affected zone of the base metal.
In other words, since this method enables reduction of the heat input applied temporarily to the material to be welded, the cooling speed is rapid at the weld section and the heat affected zone thus be narrowed. In addition, because the coarse ferrite structure formed in the preceeding weld metal and in the heat affected zone can be refined by the arc heat applied at the time of the formation of a subsequent layer, this method is advantageous also for the purpose of enhancing the low temperature toughness of the heat affected zone itself. In this instance, it is also possible to enhance the toughness of the weld metal itself by admixing proper amounts of alloy components from a powder flux or from a wire. According to this method, however, the number of passes must be increased so as to restrain the heat input per layer at a low level so that removal of slag, which is formed on the bead surface per pass, becomes extremely troublesome and time-consuming, thus resulting inevitably in the lowering of the welding efficiency. If the groove angle is narrowed in order to reduce the number of passes, removal of the slag for the first or second layer becomes all the more difficult and the object of improving the efficiency can not be accomplished.
Removal of the slag by peeling is an essential requirement for, even if welding is attempted onto the slag without removing the same, no arc would be generated and welding itself would then not be feasible because the slag layer, which has hardened and lost its conductivity, functions as a resistor. Removal of the slag per layer could be eliminated and the efficiency of the welding work could be enhanced if a multiple electrode welding method is employed in which arc an is generated from a subsequent electrode while the slag at the preceding weld section still remains molten or semi-molten (that is, in the state where the slag still maintains its conductivity). In this case, however, the distance between the preceding electrode and the subsequent electrode must be shortened so that the distance between the preceding molten pool and the subsequent molten pool becomes so small as to be inevitably adjacent to each other whereby the effect of dispersion of the welding heat input is reduced. Thus, it becomes impossible to accomplish the original aims of reducing the heat affected zone and preventing the growth of the coarse ferrite structure, and to obtain satisfactory low temperature toughness.
With the background as described above, the inventors of the present invention have conceived that the most ideal welding method would be one that enables altogether eliminating removal of the slag, diminishing the heat affected zone and preventing growth of the coarse ferrite structure (improvement of the low temperature toughness) simultaneously in a multilayer build-up welding or a multiple electrode welding in accordance with the submerged arc welding method, and have made intensive studies in search for such welding method that would provide enormous practical advantages. As a results, the inventors of the present invention have come to believe that these objects can be accomplished by furnishing the hardened slag layer with such conductivity so as to allow the generation of arc. Thus, the present invention is completed on the basis of the abovementioned concept.